Aminopterin, or N-4-[[2,4-diamino-6-pteridinyl)-methyl]amino]benzoyl]-L-glutamic acid, is a potent antifolate [see Franklin, U.S. No. Pat. No. 2,575,168]. Synthesized in 1946 by the American Cyanamid Co. (later the Lederle Laboratories), aminopterin was one of the first antifolates developed and the first to demonstrate significant clinical efficacy [see Seeger et al., J. Am. Chem. Soc. 69:2567, 1947 and Farber et al., N. Engl. J. Med. 238:787, 1948]. Other structurally related antifolates, including methotrexate, were described by the Lederle Laboratories in the ensuing years between 1948 and 1955. Compared to all other antifolates, aminopterin bears the closest structural similarity to folic acid, differing from the natural substrate by only two atoms.
Aminopterin was used clinically as a single agent in the 1940s and 1950s for the treatment of acute leukemia, psoriasis and arthritis in humans [see Farber et al., N. Engl. J. Med. 238:787, 1948; Gubner, Arch. Derm., Chicago 64:688, 1951; Rees et al., Arch. Derm., Chicago 90:544, 1964; and Gubner et al., Am. J. Med Sci. 22:176, 1951]. The prior art provides no teachings on the use of aminopterin to treat cancer, leukemia, arthritis psoriasis and other inflammatory disorder using modern combination therapy. Based mainly on the results from animal studies and anecdotal human experience, its clinical use ceased in the mid-1950s when aminopterin was determined to have inferior pharmacologic properties to methotrexate.
In particular, the prior art teaches that methotrexate has a less variable toxicity and a greater therapeutic index than aminopterin [see Burchenal et al., Cancer 2:113, 1949; Farber et al., Advances in Cancer research, pp 2-73. New York: Academic Press, 1956; Dacie et al., B. M. J. 1:1447, 1950; Sacks, M. S. et al., Ann. Intern. Med. 32:80, 1950; Goldin et al., J. Natl. Cancer Inst. 5:1657, 1955; and Glode et al., Cancer Res. 39:3707, 1979]. In other teaching methotrexate is suggested to be less toxic and efficacious than aminopterin in treating psoriasis, however the doses compared were not equipotent to one another, with methotrexate being used in an amount 4-fold less than would be required to be equipotent with aminopterin [Rees and Bennett, Arch. Dermatol. 83:970-72, June 1961; and Strakosch, Dermatologica 126:259-267, 1963]. Thus, by modern standards no conclusions could be drawn regarding their relative efficacy, toxicity, or therapeutic index. Later, Rees et al. suggest the opposite, that methotrexate may be more safe and efficacious than aminopterin in treating psoriasis [Rees et al., Arch. Dermatol. 90:544-52, December 1964]. Accordingly, the art abandoned aminopterin for methotrexate around 1955, and methotrexate has since become the standard antifolate used in the treatment of neoplastic and inflammatory disorders that include, but are not limited to leukemia, breast cancer, squamous cell tumors of the head and neck, choriocarcinoma, psoriasis, asthma, and arthritis [see Piper and Montgomery, U.S. Pat. Nos. 4,077,957 and 4,079,056].
Despite the widespread clinical use of methotrexate, there remains several shortcomings with its use as a human therapeutic. First, at oral doses greater than about 7.5 mg/m2, the bioavailability of methotrexate is 44% or less [see Balis, et al., Cancer Res. 43(5):2342, 1983; Balis, et al., Blood, 92(10):3569, 1998; Kearney, et al., Cancer Chemother. Pharmacol. 3(2):117, 1979; Pinkerton, et al., Br. J. Cancer 45(2):300, 1982; and Pinkerton, et al., Cancer Chemother. Pharmacol. 10(1):36, 1982]. Above a dose of about 15 mg/m2, absorption from the gastrointestinal tract is saturable, such that if the dose of methotrexate is increased, the fraction absorbed declines [see Balis, et al., J. Clin. Oncol., 6(12):1882, 1988 and Campbell, et al., Cancer Treat. Rep., 69(7-8):833, 1985]. Second, the bioavailability of an oral methotrexate dose greater than about 7.5 mg/m 2 is highly variable both in the same patient and between different patients, with peak plasma concentrations occurring from 0.5 to 5 hours after oral administration, and the percentage of a dose absorbed ranging from 5% to 97% [see Balis, et al., Cancer Res. 43(5):2342, 1983]. Third, a significant fraction of the population does not respond to methotrexate treatment in combination therapy [see Wallace, Clin. Exp. Rheumatol., 17:499, 1999; Ravelli and Martini, J. Rheumatol. 27(8):1830, 2000; and Campbell, et al., Cancer Treat. Rep., 69(7-8):833, 1985]. Finally, patient compliance, particularly in pediatric patients, can be problematic due to the large number of tablets required in a particular methotrexate dosage.
It is known in the art that aminopterin has greater oral bioavailability than methotrexate in adults (˜85%), has efficacy as a single agent in some patients with T-lineage leukemia refractory to conventional therapy employing methotrexate, and is at least about 10 times more potent than methotrexate [see Ratliff et al., J. Clin. One. 16:1458, 1998; Glode et al., Cancer Res. 39:3707, 1979; Cole et al., Proc. Am. Assoc. Cancer Res. 43:749, 2002; and Sirotnak and Donsback, Cancer Res. 32:2120, 1972]. However, the prior art provides no teachings on the oral bioavailability of aminopterin in pediatric patients, variability of oral bioavailability in the same patient, variability of oral bioavailability in different patients, or the efficacy of aminopterin in treating a variety of cancers or inflammatory diseases using modern combination therapy.
The prior art also teaches that any utility aminopterin might have is mitigated by a more narrow therapeutic index and more variable toxicity relative to methotrexate [see Burchenal et al., Cancer 2:113, 1949; Farber et al., Advances in Cancer research, pp 2-73. New York: Academic Press, 1956; Dacie et al., B. M. J 1:1447, 1950; Sacks, M. S. et al., Ann. Intern. Med. 32:80, 1950; Goldin et al., J. Natl. Cancer Inst. 5:1657, 1955; and Glode et al., Cancer Res. 39:3707, 1979]. Accordingly, there is also a need in the art for an antifolate alternative to methotrexate which compared to methotrexate, has an equivalent or greater therapeutic index, and an equivalent or smaller coefficient of variation of toxicity.
It is known that methotrexate is metabolized to various polyglutamated species and that the number of glutamates in the polyglutamate chain of a particular species is proportional to its efficacy in cell-based and enzyme systems [see Allegra, et al., Proc. Natl. Acad. Sci. U.S.A., 82:4881, 1985; Sirotnak and Donsback, Cancer Res. 32:2120, 1972; and Ratliff et al., J. Clin. Oncol. 16:1458, 1998]. Thus the art suggests that longer polyglutamate chains should improve clinical efficacy of an antifolate. However, there are no teachings that this is relevant clinically, or whether another combination of species, including those with polyglutamate chains with fewer glutamates, might provide for an antifolate with greater efficacy.
Rees et al. noted in 1964 that aminopterin is even less pure than methotrexate and that instability was a problem [Rees et al., Arch. Dermatol. 90:544-52, December 1964]. It is also known that the aminopterin within pharmaceutical compositions of the prior art were made via a one-pot aqueous condensation of a pteridine with p-aminobenzoic acid and glutamate, and contained variable amounts of impurities, mostly contaminating folic acid, that represented up to 41 weight percent of a preparation [see Franklin, U.S. Pat. No. 2,575,168; Seeger, et al., J. Am. Chem. Soc. 69:2567, 1947; Seeger, et al., J. Am. Chem. Soc. 71:1753, 1949; Sirotnak and Donsbach, Biochem. Pharmacol. 24:156, 1975; Loo, J. Med. Chem. 8:139, 1965; Heinrich et al., J. Am. Chem. Soc. 75:5425; Weygand et al., Naturforsch. 6b: 174, 1951; Hutchinson and Burchenal, Proc. Am. Assoc. Cancer Res. 1:26, 1953; Waller, et al., J. Am. Chem. Soc. 70:19, 1948; and Hultquist and Dreisbach, U.S. Pat. No. 2,443,165]. The prior art teaches that the simultaneous co-administration of folic acid and aminopterin had no effect on toxicity, the predictability of toxicity, or the therapeutic index of aminopterin [see Nichol and Welch, Proc. Soc. Exp. Biol. Med. 74:403, 1950; Golden et al., Cancer Res. 13:843, 1953; Greenspan et al., Cancer 3:856, 1950; Franklin et al., Proc. Soc. Exp. Biol. Med. 5:1, 1948; Schoenback et al., J. A. M. A. 144:1558, 1950; Dameshek, Blood 4:168, 1949; Farber, Blood 4:160, 1949; and Dameshek et al., Blood 5:898, 1950].
There are no teachings in the prior art on how to prepare pharmaceutical compositions having greater aminopterin purity or consistent aminopterin purity, or how to use such compositions therapeutically to obtain greater interpatient oral bioavailability in pediatric patients, a smaller interpatient coefficient of variation of oral bioavailability, a smaller mean intrapatient coefficient of variation of oral bioavailability, a greater therapeutic index, a smaller coefficient of variation of toxicity, efficacy in combination therapy, and efficacy of certain polyglutamated metabolites.